CAMP FOSTER, Okinawa — Japanese government officials have chastised their U.S. allies after Marine Corps helicopters reportedly flew over an Okinawan elementary school where a CH-53E Super Stallion’s window fell last month.

A UH-1Y Venom and two AH-1Z Vipers were spotted above Futenma Daini Elementary School on Thursday afternoon, said a spokesman for the Okinawa Defense Bureau, which is part of Japan’s Defense Ministry. The school is adjacent to Marine Corps Air Station Futenma.

Students were in class at the time, school officials said.

“It’s deplorable that this has happened even though we’ve requested firmly that the U.S. not fly above the school,” Chief Cabinet Secretary Yoshihide Suga told reporters Thursday. On Friday, he said he would continue to urge the U.S. to not fly over the school.

Officials had been watching for military aircraft over Okinawan schools after a Dec. 13 incident in which a Super Stallion’s window inexplicably became separated from the aircraft and landed on a Futenma Daini sports field while schoolchildren were playing.

Footage from security cameras set up after that incident reportedly caught Thursday’s flyover.

Speaking to reporters Thursday, Defense Minister Itsunori Onodera called the flyover “extremely regrettable” and said he had lodged a complaint with U.S. Forces Japan’s deputy commander, Maj. Gen. Charles Chiarotti.

Onodera said the U.S. military denied the flyover took place when asked about it by his office. Marine officials did not respond to Stars and Stripes’ requests for comment on Friday.

Onodera added that he thought Martinez and Secretary of Defense Jim Mattis — who apologized to Japan earlier this month for recent aircraft mishaps — understood that refraining from flying over Okinawan schools is an important issue.

Onodera said he would urge the U.S. military to instruct all units not to fly over the elementary school and ask for specific reasons for Thursday’s flyover, a Defense Ministry spokesman said.

Okinawa’s anti-base Gov. Takeshi Onaga said he was outraged by the incident, according to the Asahi newspaper. Onaga plans to visit Tokyo to lodge a protest with the Japanese government.

Officials from the Ginowan city board of education said they have had lingering safety concerns after the window fell, and had asked the U.S. military on Okinawa not to fly over the school, a spokesman said.

The school held an evacuation drill before the flyover Thursday and hasn’t used the playground since the window fell.

“We want to be able to hold classes in a normal and quiet environment,” the spokesman said. “We just want them to not fly. That is all we want.”

Thursday’s flyover is the latest aviation mishap to strain relations between the two allies regarding the U.S. military’s presence on the southern island prefecture.

The falling-window incident on Dec. 13 was followed by emergency landings by Marine helicopters on Jan. 6 and Jan. 8. On Dec. 7, a plastic part thought to belong to a U.S. military helicopter was found on the roof of an Okinawan day care facility.

In October, a 1st Marine Aircraft Wing Super Stallion made an emergency landing in a farmer’s field outside Okinawa’s Northern Training Area after an inflight fire. The aircraft was an almost total loss.

The incidents stand to embolden Okinawa’s small but potent protest movement, which is backed by Onaga, and could lead to renewed demonstrations against U.S. military aircraft and plans to relocate Marine air operations within the prefecture, from Futenma to Camp Schwab in Okinawa’s remote north.

This image from an NHK broadcast shows a window from a CH-53E Super Stallion helicopter after it fell onto an elementary school sports field near Marine Corps Air Station Futenma, Okinawa, Wednesday, Dec. 13, 2017.SCREENSHOT FROM NHK